NEARFIELD DETECTION OF DIPOLE SOURCES BY THE GOLDFISH (CARASSIUS-AURATUS) AND THE MOTTLED SCULPIN (COTTUS-BAIRDI)

Surprisingly few behavioral data exist on dipole source detection by f ish, despite the fact that dipole sources more closely approximate bio logically relevant signals than do more nearly monopole sources such a s loudspeakers, the stimulus used in nearly all fish auditory studies. In this study, dipole source detection is investigated for two fish s pecies that differ in both their auditory and lateral line systems, th e two systems capable of detecting dipole sources. Conditioned suppres sion of respiration in the goldfish and an unconditioned orienting res ponse in the mottled sculpin were used to measure detection of a 6 mm diameter, sinusoidally vibrating sphere as a function of vibration fre quency and source distance. Sound pressure thresholds for the goldfish were nearly independent of distance (15-60 mm) at 800 Hz, but increas ed with distance at 50 Hz, as they did for the mottled sculpin. The sl opes of 50 Hz source level-distance functions, however, differed betwe en the two species. Slopes for the goldfish were independent of distan ce, remaining at around 8 dB per distance doubling, which is near the 6 dB per distance doubling measured for sound pressure attenuation awa y from the source, but less than the 18 dB per distance doubling for i ncompressible flow, measured with an anemometer. Those for the mottled sculpin increased with increasing distance, approaching 18 dB per dis tance doubling. The nonlinear increase in source level necessary to re ach threshold detection was quite similar to the nonlinear decrease in incompressible flow levels measured with the anemometer. Nonlinear in creases with distance for 50 Hz sources near the trunk of the mottled sculpin were also similar to those near the head of the fish, where ch anges in source frequency had little effect on source level-distance f unctions. These results indicate that sound pressure detection by the ear is important for dipole detection by the goldfish, but that incomp ressible flow detection by the lateral line is more important for the mottled sculpin. They also indicate that fish such as the goldfish, wi th a pressure-sensitive swimbladder, are capable of detecting dipole s ources at greater distances than are fish without such structures.